White voters often tell pollsters they support black candidates, but they don't always follow through.

Is Harold Ford Jr. really doing as well as the polls suggest? Is he conceivably on his way to becoming the first black Southern senator since Reconstruction? The answer may well be yes, but Ford can hardly take that for granted. As black candidates reaching out to largely white constituencies have discovered in the past, when it comes to measuring political popularity there are lies, damned lies—and polls, on which they rest their fate at their peril.

The phenomenon was first widely noted in 1982, when Los Angeles Mayor Tom Bradley lost a squeaker of a race for governor after being widely projected as the winner. Douglas Wilder also came up against the "Bradley Effect" when he barely won the 1989 contest for governor of Virginia, after leading comfortably in the polls.

Ronald Walters of the University of Maryland was at Wilder's hotel as a projected easy victory turned into a nail-biter. That is a night "I'll never forget," says Walters, who thinks it "naive" to believe that things have changed very much. He believes that some percentage of whites—perhaps 5 percent or so, intent on being seen as less biased than they may be—will claim to support a nonwhite candidate when they actually do not.

Other political observers think the effect may have diminished over time. "We may be seeing the turning of this," says Ed Sarpolus, vice president of EPIC-MRA, a Michigan-based polling firm.